The major objective is to determine if the chromatin-bound cholesterol is involved in the control of synthesis of the enzyme, 3-hydroxy-3-methylglutaryl CoA reductase. The activity of this reductase is the major determinant of the rate of cholesterol synthesis in liver. The ratio of free and of esterified cholesterol to DNA will be determined in rats treated in various ways to increase and decrease reductase activity and correlations similar to those we have previously reported for the circadian rhythm will be searched for. A related objective is to determine whether the cholesterol-binding protein we have previously isolated from liver cytosol is present also in the transport of exogenous cholesterol from the plasma membrane and of endogenous cholesterol from the microsomes to chromatin. Antiserum to this protein (which is already available) will be used to investigate the proteins in the nuclear cytoplasm and in chromatin. In vitro incubations of chromatin with the cholesterol-binding protein and with lecithin-cholesterol dispersions will be done to obtain information on the exchangeability of the chromatin-bound cholesterol. Studies will also be continued on the reversible inactivation of the reductase by mevalonic lactone and on the irreversible inactivation which occurs in vivo and in hepatocytes during incubation, but not in vitro, to try to obtain a clearer understanding of the factors responsible for the extraordinarily rapid turnover of this enzyme in the intact cell. A method for artificially controlling the activity of this enzyme might provide a means of decreasing plasma cholesterol levels in humans and thus preventing atherogenesis.